Electrostatic discharges (ESD) can damage semiconductor electronic parts, rendering them inoperative. Some electronic parts are more prone to such damage than others; and electronic parts can be designed to be resistant to ESD. An objective standard for testing the sensitivity of electronic devices to ESD has been formulated and is given in the military specification MIL-STD-883C, Notice 7, Method 3015.6 entitled Electrostatic Discharge Sensitivity Classification which is hereby incorporated by reference.
A prior art circuit designed to simulate an ESD from a human body source is shown in FIG. 1. A regulated high-voltage power supply 10 charges a capacitor 12 through a switch 14 and a resistor 16. The voltage is adjustable to test various levels of ESD sensitivity and may be set from zero volts to plus or minus 10 kilovolts. After the capacitor 12 is charged, the switch 14 is operated, discharging the stored charge through a 1500 ohm resistor 18. The two contact pins of the device under test (DUT) 20 connected to the test circuit experience a high-voltage, fast rise-time signal pulse, simulating an electrostatic discharge.
The charge-discharge cycle is repeated a predetermined number of times at both positive and negative voltage polarities. For electronic devices having more than two contact pins, the series of charge-discharge cycles must be repeated for each pairing of pins. The testing procedure can take vast amounts of time for a typical multi-pin electronic device. After subjecting the electronic device to the simulated ESD, it is then tested for correct operation. Devices that fail at one voltage level may pass at a lower level. Thus, a device's resistance to ESD may be reliably quantified.
Weil U.S. Pat. No. 4,721,899 and Richman U.S. Pat. No. 4,742,427 both disclose devices which can generate high-voltage, fast rise-time pulses for testing ESD sensitivity. However, neither reference discloses a means for routing the pulses to selected pins of the DUT to speed the tedious testing procedure.
Lutz U.S. Pat. No. 4,617,542 discloses a switch for switching such signals consistently. However, the disclosed switch is intended to be used in place of switch 14 of FIG. 1. As such, its construction solves the problem of switch bounce not confronted for the present problem of routing. The routing switches may be closed and allowed to settle before activating switch 14.
In earlier reed relay switches used in ESD testing devices, the reed contacts capacitively coupled with the relay's electromagnet windings, thus decreasing the rise-time of signals passing through the switch. This rise-time problem was solved by interposing a conductive shield between the reed contacts and the windings, thus forming a coaxial transmission line. Unfortunately, the capacitance between the shield and the rest of the external circuitry forms a high quality factor (hereinafter "Q") circuit, causing the signal to oscillate or "ring."
What is needed is a switch system for routing high-voltage signals with short rise-times while maintaining consistent pulse shapes and minimizing ringing.